1. Field of the Invention
The present invention relates to filters for filtering out unwanted harmonics in power distribution systems that use variable-frequency drives (“VFDs”) for controlling the rotational speed of three-phase alternating current (AC) electric motors by controlling the frequency of the electrical power supplied to the motor.
2. Description of Related Art
Large harmonic distortions cause malfunctions of meters and relays, nuisance tripping of circuit breakers, and equipment overheating. Typical prior art single-tuned passive harmonic filters are illustrated in FIG. 3 of U.S. Pat. No. 5,444,609, which patent is incorporated herein in its entirety by reference. Such filters are tuned to a series of specific harmonic frequencies. A simplified version of FIG. 3 of U.S. Pat. No. 5,444,609 is depicted in FIG. 1 as a passive harmonic filter system 101, which includes harmonic filters 103a, 103b, and 103c, respectively tuned at harmonic frequencies of the 5th, 7th, and 11th harmonics. The passive harmonic filter system 101 also includes circuit breakers or switches 105a, 105b, 105c, and 105d which serve to switch the harmonic filters in or out of the electrical power systems.
The passive harmonic filter system 101 usually has an interlocking control (not shown) among the filters during filter operation. For example, when the filter 103a fails, the interlocking control logic turns off filters 103b and 103c in order to prevent them from overloading, and to prevent a serious resonance condition in the power distribution system. However, this leaves the power distribution system that was being protected by the passive harmonic filter system 101 without any harmonic filtering.
Although there are many designs for passive harmonic filters that are well known in the art, considerable shortcomings remain. What is needed is a passive harmonic filter system that will not fail when interlocking controls shut off individual parts of the filter system.